Cercospora species are a highly successful group of fungal pathogens that cause disease on a diversity of host plants, including corn, sugar beet, tobacco, coffee, soybean, and banana, as well as many ornamental and weed species. For example, the Cercospora species C. nicotiniae, C. arachidicola, C. zeae-maydis, C. kikuchii, C. oryzae and C. beticola are pathogenic to tobacco, peanut, corn, soybean, rice and sugar beet, respectively. The Cercospora species are aerial pathogens. In general, spores produced by these fungi germinate on leaf surfaces and then enter the leaf (i.e. through stomata). Fungal mycelium then kills leaf cells and causes severe blighting of the leaf tissue by spreading through intercellular spaces in leaf tissues. See M. E. Daub, American Chemical Society Symposium Series No. 339, pp. 271-80 (J. R. Heitz and K. R. Downum, eds., Washington, D.C., 1987). In addition to damaging leaf tissue, Cercospora species may also damage other plant tissues, such as the seed coat in soybean. Cercospora species cause major economic problems due not only to their world-wide distribution and wide host range, but also because naturally-occurring resistance to the disease has not been identified in many host species.
One of the reasons for the success of this group of pathogens appears to be their production of cercosporin, a perylenequinone phytotoxin and a photosensitizer. Cercosporin is produced by many members of the genus Cercospora, and has near universal toxicity to plants. Cercosporin is also toxic to mice, bacteria, and many fungi. See R. B. Batcharova et al., Phytopathology 82, 642-46 (1992); C. Balis and M. G. Payne Phytopathology 61, 1477-84 (1971); M. E. Daub Phytopathology 77, 1515-20 (1987); A. O. Fajola, Physiol. Plant. Pathol. 13, 157-64 (1978); S. Yamazaki et al., Agric. Biol. Chem. 39, 287-88 (1975). Cercosporin has additionally been shown inactivate protein kinase C and to be cytotoxic to human tumor cells. See T. Tamaoki and H. Nakano Bio/Technology 8, 732-35 (1990). These observations suggest that cercosporin has almost universal toxicity to cells, and that resistance is due to active defense mechanisms present in the few resistant organisms. Production of cercosporin appears critical for successful pathogenesis, as fungal mutants deficient in cercosporin synthesis are unable to parasitize their host plants (R. G. Upchurch et al., Appl. Envir. Biol. 57, 2940-45 (1991)).
The presence of light has been shown to be critically important in the development of disease symptoms on hosts susceptible to Cercospora infection, and symptom development in infected plants is enhanced by high light intensities. See, e.g., M. E. Daub and M. Ehrenshaft, Physiol. Plant 89, 227-36 (1993); L. Calpouzos, Ann. Rev. Phytopathol. 4, 369-390 (1967). The role of cercosporin as a photosensitizer is thus related to its ability to cause toxicity and injury in cells. Although cercosporin was the first toxin synthesized by plant pathogens to be recognized as a photosensitizer, numerous other plant pathogenic fungi also produce perylenequinone toxins and other compounds that are photosensitizers. The production of photoactivated perylenequinones by such a diversity of plant pathogens suggests that photosensitization may be a more common plant pathogenesis factor than has been previously recognized.
Virtually the only organisms which show resistance to cercosporin are the Cercospora fungi themselves, and some related fungi that produce similar toxins. Attempts to obtain resistant plants and fungi through mutagenesis and selection of cells in culture have not been successful. M. Ehrenshaft et al., (Phytopathology 86, S11 (Abstract 93A) (Supplement 1996)) report using a wild type C. nicotianae genomic library to isolate cosmid clones that complement two classes of C. nicotianae mutants which are sensitive to cercosporin. Transformation of the mutants with one of the clones restored wild type resistance to cercosporin and other photosensitizers. However, isolation and sequencing of the specific genes responsible for the resistance to the photosensitizers is not described therein.
It would thus be highly desirable to isolate genes which encode resistance to photosensitizers such as cercosporin. Moreover, it would be desirable to provide plants and other organisms resistant to diseases caused by Cercospora species and other pathogenic fungi that produce photosensitizers.